“Don’t Think Twice” is a comedy so truthful and cutting it makes you want to curl up in a ball, cover your eyes with your hands and watch it through the cracks of your fingers.

That’s a good thing. In fact, it’s a great thing. Writer-director-star Mike Birbiglia’s insightful look at the world of improv comedy — its form, as well as the strange birds who perform it — could stand as the definitive film about art form, and that’s not just because it’s one of the few (if any) films made about it.

“Don’t Think Twice” burrows into the hearts and minds of a group of improv comedians, exposing their hopes, fears, insecurities, anxieties and jealousies, with rare and often uncomfortable levels of honesty. To watch them is to know them, and Birbiglia (“Sleepwalk With Me”) knows these characters up and down, in and out, backward and forward.

Keegan-Michael Key plays Jack, the leader of improv troupe the Commune, who face an uncertain future due to a looming lease termination at their longtime residence. Meanwhile, he lands a gig at a “Saturday Night Live”-like sketch show called “Weekend Live,” while the rest of the group — which includes Miles (Birbiglia), Bill (Chris Gethard) and his girlfriend, Samantha (Gillian Jacobs) — is left sitting on the sidelines.

They’re happy for their friend, right? Well, not so much as they are bitter they didn’t get the call, as Birbiglia wades through the layers of envy and selfishness that drives comedians, and if we’re real, all of us. “Don’t Think Twice” is laugh-out-loud funny and unflinchingly human. Don’t think twice, just see it.